With regard to uplink enhancements for dedicated transport channels several techniques are currently being discussed for standardization in 3GPP, among them several retransmission functionalities, including so called Node B controlled Hybrid Automatic Repeat reQuest (HARQ). In such a scheme, a mobile unit or user equipment (UE) transmits data units (DU) in the uplink and the Node B or base station tries to decode each of them and requests retransmissions of erroneously received data units. In addition, the Node B may optionally combine received soft information from multiple transmission attempts to further increase the performance.
Wideband Code Division Multiple Access (WCDMA) systems use a concept of transport channels, where the attributes of each transport channel determine the manner and characteristics for transmitting data over the air interface. One important characteristic of a transport channel is the transmission period or Transmission Time Interval (TTI), which denotes the period of time over which coding and interleaving is performed. Currently, TTI:s of 10, 20, 40, and 80 ms are supported in the uplink, although additional values may be introduced in the future. On each transport channel, data arrives in form of one or several transport blocks per TTI. Typically, multiple transport channels are configured in a user equipment, by the radio network controller (RNC), in order to support a mix of different services; however, multiple transport channels may notwithstanding also be configured for a single service. The transport channels are coded and multiplexed into a single Coded Composite Transport CHannel (CCTrCH), which is subsequently transmitted over the air interface from the UE to the Node B. Rate matching is used to balance the quality requirements between different transport channels. The multiplexing of multiple transport channels is schematically illustrated in FIG. 3.
Retransmission functionalities, such as for example so called Hybrid ARQ, if introduced, cannot operate on the CCTrCH since not all services, e.g. voice, allow for retransmissions. Hence, the hybrid ARQ mechanism is preferably defined per transport channel. Furthermore, if so-called soft combining is supported, the corresponding ARQ mechanism has to be located between the coding and transport channel blocks. In FIG. 3, the hybrid ARQ mechanism precedes the rate matching (RM) block. However, it is equally possible for the RM block to precede the ARQ mechanism, or be merged together to constitute one single block
Hybrid ARQ is currently not used for the uplink of a WCDMA-based communication system. For the HS-DSCH downlink in WCDMA, where hybrid ARQ is used, only a single transport channel with a fixed 2 ms TTI is used.